1. Field of the Invention
The present invention relates to a layered structure comprising an insulator thin film and an oxide superconductor thin film, more specifically, to a layered structure comprising an insulator thin film and an oxide superconductor thin film with a little interdiffusion at an interface between those two thin films.
2. Description of Related Art
Oxide superconductors which have been recently advanced in study have higher critical temperatures than those of metallic superconductors, so that they are more applicable for practical usage. For example, Y--Ba--Cu--O type oxide superconductor has a critical temperature higher than 80 K and it is announced that Bi--Sr--Ca--Cu--O type oxide superconductor and Tl--Ba--Ca--Cu--O type oxide superconductor have critical temperatures higher than 100 K.
In order to apply the oxide superconductors to superconducting devices, it is necessary to make oxide superconductors in a form of thin films. The term thin films means films, having a thickness of from 10.sup.-1 nanometers to on the order of 10 .mu.m, artificially deposited on substrates. The oxide superconductor thin films are usually deposited on single crystal substrates, for example a MgO substrate, a SrTiO.sub.3 substrate, a YSZ (yttrium stabilized zirconia) substrate, etc., by various methods such as a sputtering, an MBE (molecular beam epitaxy), a reactive co-evaporation, a CVD (chemical vapor deposition). The oxide superconductor thin films grow epitaxially on these substrate.
It is also necessary to deposit other thin films on the oxide superconductor thin films in order to manufacture some types of superconducting devices. For example, a tunnel type Josephson junction comprises a layered structure of a first superconducting layer, an thin insulator layer and a second superconducting layer stacked in the named order. Therefore, if an oxide superconductor is applied to the tunnel type Josephson junction device, a first oxide superconductor thin film, an insulator thin film and a second oxide superconductor thin film should be stacked in the named order.
A superconducting field effect device, a candidate of realistic three-terminal superconducting devices, has a superconducting channel and a gate electrode formed on the superconducting channel through a gate insulator layer. If a superconducting field effect device is manufactured by using an oxide superconductor, it is necessary to stack an oxide superconductor thin film, an insulator thin film and a normal conductor film in the named order.
In addition, in order to fabricate a superconducting multi-layer wiring structure by using an oxide superconductor, oxide superconducting thin films and insulator thin films should be alternately stacked.
All of above superconducting devices and elements require high quality thin films. Namely, oxide superconductor thin films and other thin films preferably should have excellent properties originated by their high crystallinity. It is more preferable that the thin films are formed of single crystals. If one of the thin films is polycrystalline or amorphous, the device or the element may have degraded characteristics or may not even operate.
Additionally, in the above superconducting devices and elements, interfaces between the two different thin films are also important. It is preferable that there is no interdiffusion through the interfaces, therefore, there should exist clear boundary planes. In particular, if constituent elements of the nonsuperconductor thin films diffuse into the oxide superconductor thin films, superconducting properties of the oxide superconductor thin films are fairly degraded, and vice versa.
In a prior art, all the thin films consisting of the layered structure are deposited by using the same apparatus in order to prepare a layered structure having clear interfaces. For example, in case of reactive co-evaporation using Knudsen cell (K cell) evaporation sources, an oxide superconductor thin film is deposited on a substrate at first, and then, the other evaporation sources are used so as to deposit an insulator thin film on the oxide superconductor thin film successively. A surface of the oxide superconductor thin film, namely an interface between the oxide superconductor thin film and the insulator thin film, is not exposed to the air so as to be prevented from contamination and degradation.
However, during the above process, the insulator thin film should be deposited at a suitable substrate temperature for forming a highly crystallized and, therefore, high quality insulator thin film. At this relatively high substrate temperature, significant interdiffusion is caused at the interface, mainly because of its long deposition time. By this, both the oxide superconductor thin film and the insulator thin film are degraded so as to have poor qualities or properties.
If the insulator thin film is deposited at a lower substrate temperature in order to avoid the interdiffusion at the boundary plane, the insulator thin film is less crystalline or may be formed of amorphous so as to have degraded properties.